Comments by Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 18 January 2016

    Your math about how long the batteries would last is fine, but the point is you should avoid putting your battery packs in parallel. You should try to use bigger cells if you really need the 5Ah in one pack, or you could put three fans on each pack and have two parallel systems.

    If you really want a single pack of eight AA cells, you could put them all in series to make a 9.6V pack, then put pairs of fans in series and power them from the higher voltage. I don't know about the details of the fans, so this could be tricky in the same way that putting batteries in parallel is tricky: it's difficult to ensure the power will get shared evenly. For instance, if the fan is just a motor and you stall one of the series pair, the voltage across it would go to 0V and the other fan would get the full battery voltage of around 9.6V, which might damage it.

    - Jan

  • On losing my baby

    On losing my baby

    - 31 December 2015

    It got a lot better, but now it's getting worse as we approach his birthday. There are so many triggers for those sad memories. We haven't made much progress on the core conflicting goals of not being sad and not forgetting him.

    Thanks for your thoughts.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 15 December 2015

    Amps multiplied by volts gives you watts. For the same current (amps), higher voltage gets you more power (watts). Factoring in the time (hours) just takes you from an instantaneous rate (watts) to total capacity or energy (watt-hours).

    An easy way to see this intuitively is if you think of two 6V, 51Ah batteries. You would need two of them in series to get to 12V and 51Ah. A single battery has 6V times 51Ah = 306Wh of energy in it. If you have two of the batteries, you have twice as much energy stored, which is also what you get when you multiply 12V by 51Ah to get 612Wh.

    - Jan

  • Continuous-rotation servos and multi-turn servos

    Continuous-rotation servos and multi-turn servos

    - 5 December 2015

    Hello, juerg.

    I addressed the reasons for some of these hobby servo limitations in the first post of this series, Introduction to servos (see the "Ramifications of the servo’s intended use" section):

    https://www.pololu.com/blog/12/introduction-to-servos

    The short answer is that the applications these hobby servos are designed for do not need the features you are looking for, and adding them would make them much more expensive. The main feature most of us want from a servo is to hold absolute position, and incremental encoders are not enough if you don't have the option to go to some home position as part of a power-up sequence. High-resolution, absolute position encoders are expensive, and even then, how would the servo know if it is at, say, 10 degrees or 370 degrees when it turns on?

    There are some fancier servos for robots that might work for your application. For instance, the Dynamixel servos specify a 300 degree position feedback range and also have a continuous rotation mode. I have not played with one and don't know how easy it is to transition between modes and how it behaves when you do, but maybe that could work for you if you never need to do position control within that 60-degree blind region. I suspect there are some robot hobby servos that have continuous positioning over the full 360 degrees, but I do not specifically know of one.

    You might also get a decent servo that will get you at least 150 degrees or so and then use an external 1:3 gearbox to get triple the range (and 1/3 the torque and resolution). Servo City has some external gearboxes for servos, though they might mostly be intended for increasing torque, not reducing it and increasing range.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 27 August 2015

    Some of the comments and questions have been tending toward subjects like chargers that are beyond the scope of this discussion, which is supposed to be about understanding units for battery capacity such as Ah and Wh. Unfortunately, I do not have time to be answering all these questions, so I will generally not respond to questions about particular battery-related products.

    - Jan

  • New products: Magnetic quadrature encoders for micro metal gearmotors

    New products: Magnetic quadrature encoders for micro metal gearmotors

    - 27 August 2015

    Unfortunately, we were not able to get the magnetic field strong enough with more segments on that small of a disc. We might still try with a larger disc, but that makes it less appealing for us since it wouldn't be a drop-in replacement in applications like the Zumo 32U4, plus the larger size would mean more new molds to make, so it's not a high priority project right now.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 28 July 2015

    Steve,

    Wow, that looks like a fun RC vehicle! The battery is probably there more to stabilize the voltage, not the current in your system. How that system works depends on the interactions of your alternator, battery, and motor controller, so I can't give you an answer based on battery capacity alone, especially because you probably have concerns beyond it working once (e.g. how long will the battery last if you use it this way?). This battery manual from Yuasa has some nice general info:

    http://www.yuasabatteries.com/pdfs/TechManual_2014.pdf

    There's a graph on page 8 that suggests you should generally be able to get 100 A even out of a crappy 10 Ah battery. That's just for 30 seconds, with the voltage allowed to drop as low as 7.2V, but if the battery alone can do that, I think it should be able to serve your purposes. Again, I don't know what that will do for the long-term life of the battery, but we just got some 12 V, 12 Ah batteries for $25 each, so you'd only be risking about $50 while taking off 75% of your battery weight. You can test how well they're serving their purpose by monitoring the stablility of your battery voltage under different loads.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 16 June 2015

    Kris,

    600 mAh is not much for a AA battery (you sure you don't mean AAA?). A higher-capacity battery with the same voltage won't blow out your LED, but you might not necessarily get anything more out of it since your solar light might be limited elsewhere. For instance, the solar panel and charge circuitry in there might need 18 hours of strong sunlight just to fully charge the 600 mAh battery, in which case you probably never even fully charge it. If you're only putting 400 mAh or so into your 600 mAh battery, you'll still only end up with the same 400 mAh in there even if you go to a 900 mAh battery.

    Separately, it's not generally the case that a system designed for one battery will automatically work with a bigger one. But the solar light is probably just trickle-charging the battery, and if you have one of the same chemistry and approximately the same capacity, it will probably be fine. However, going to something like 60 mAh or 6000 mAh (10x smaller or bigger) is more likely to result in problems that could damage the battery or your other circuitry.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 16 June 2015

    Mark,

    Did you even read the original post? It's about how A and Ah are related. Anyway, if your 2 Ah battery lasts about 6 minutes (1/10 of an hour), your motor draws about 20A. If the battery lasts 12 minutes, the motor draws about 10A. These would just be the average current over that discharge time, and the motor might draw much more when under heavy load or when it's starting up. You could try the 10A supply you have and see if it's enough, but it's quite possible you would need a much beefier power supply to match what the battery can deliver.

    - Jan

  • New product: Logic Level Shifter, 4-Channel, Bidirectional

    New product: Logic Level Shifter, 4-Channel, Bidirectional

    - 1 June 2015

    Nick,

    Is there a specific parameter you are concerned about? We do not want to commit to particular components for boards like these, but the specs should be in line with typical components like this (dual low-voltage MOSFET in a small package).

    - Jan

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